feat: parse oracle-url from YAML order config (Phase 6)

[hardyjosh]

Caution

Merge order: Merge #2459 first, then this PR.

Motivation

Orders that use signed context oracles need a way to specify the oracle endpoint URL in the dotrain YAML config. This is the missing link between the YAML front matter and the on-chain metadata that takers use to discover oracles.

Solution

• OrderCfg: new oracle_url: Option<String> field
• YAML parsing: oracle-url parsed via optional_string (same pattern as deployer/orderbook)
• ALLOWED_ORDER_KEYS: added oracle-url
• When an order with oracle-url is deployed via addOrder, a SignedContextOracleV1 item is included in the order's RainMetaDocumentV1

orders:
  my-order:
    oracle-url: https://rain-oracle-server.fly.dev/context
    inputs:
      - token: base-weth
    outputs:
      - token: base-usdc

Checks

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• made this PR as small as possible
• unit-tested any new functionality
• linked any relevant issues or PRs
• included screenshots (if this involves a front-end change)

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